A biaxially stretched polypropylene film is used for various uses such as packaging use, taping use and electrical use of cable wrapping or capacitor production, because of its excellent transparency, mechanical properties and electric characteristics.
As to capacitor production, biaxially stretched polypropylene films having excellent withstanding voltage characteristics and low loss characteristics are desirably used to make a high voltage capacitor for direct current and alternate current.
Recently, electric equipment has been controlled with inverters so that a downsized capacitor having a high capacitance has been further required. Under such a requirement, specifically in the automobile market including the hybrid car market, the biaxially stretched polypropylene film has been required to be thinner together with improving withstanding voltage, security (self-healing ability) and element processability.
From viewpoints of withstanding voltage, security and element processability, the biaxially stretched polypropylene film has to have a proper rough surface. That is important specifically to improve film slipping and oil impregnation, as well as security of deposition capacitor. The security of a metal deposition capacitor having a metal deposition membrane as electrode formed on the dielectric film is a helpful function even from a viewpoint of safety to recover insulation by deposited metal scattering with electric discharge energy under abnormal discharging so that short circuiting is prevented to maintain capacitor functions.
To make such a rough surface, they have suggested a mechanical method such as embossing method and sand blasting method, a chemical method such as chemical etching with a solvent, a method of stretching a sheet made of blended polymers of polyethylene or the like, and a method of stretching a sheet containing β crystals, as disclosed in JP-S51-63500-A and JP 2001-324607-A.
However, the mechanical and chemical methods might decrease the roughness density while the method of stretching a sheet containing β crystals might generate coarse protrusions. Thus, the above-described conventional methods might not achieve sufficient characteristics such as roughness density, coarse protrusion and the number of protrusions. Further, films having the rough surface made by such a method tend to partially be impregnated with insufficient oil impregnation between film layers in producing a capacitor so that the lifetime of the capacitor is shortened. Furthermore, although the method of stretching a sheet made of blended polymers of polyethylene or the like could produce a capacitor with less air bubbles, the blended polymers might deteriorate recyclability.
Furthermore, the biaxially stretched polypropylene film made by any of the above-described methods might have a problem of reliability and lifetime with insufficient withstanding voltage and security under extremely severe capacitor use condition of electric potential gradient of 400V/μm or higher. The electric potential gradient is defined as an applied voltage per unit film thickness, to be calculated by dividing a voltage applied to a dielectric film by the film thickness.
To improve the uniformity of protrusions and roughness density, JP 2011-122143-A, WO 2012/002123 and JP 2011-122142-A suggest films to which high melt tension polypropylene is added. JP 2011-122143-A and WO 2012/002123 disclose effects in the uniformity of protrusions and roughness density, but fail to precisely control protrusion height on both film sides. Thus the requirements of high withstanding voltage, security and element processability for the automobile use could not be satisfied sufficiently.
JP 2011-122142-A discloses a biaxially stretched polypropylene film and producing method thereof having a desirably controlled roughness on both film. However, the technique disclosed in JP 2011-122142-A could hardly be applied to thin films and fails to achieve a sufficiently high withstanding voltage because the technique targets comparatively thick films. Further, it is difficult for the technique to make both film sides microscopically rough and to control protrusion height on both film sides.
JP 2007-308604-A and JP 2008-133446-A disclose methods of forming a microscopic rough surface by designing β crystal fraction of a cast stock sheet within a predetermined range to balance the element winding property with withstanding voltage, under a prescribed roughness on at least one film side. However, such a method could not control the surface roughness on both film sides and the obtained microscopically rough film surfaces could not satisfy the high withstanding voltage, security and element processability required for the automobile use.
It could therefore be helpful to provide a biaxially stretched polypropylene film, metallized film and film capacitor, capable of achieving excellent withstanding voltage, security and reliability as securing stable element processability.